Recently, along with downsizing of an electric device, an electronic device, or the like, a compact and high density multilayer wiring board or the like to be mounted on these devices has been demanded. As this multilayer wiring board, a wiring board that is fabricated with what is called a build up method where a plurality of resin insulating layers and a plurality of conductor layers are alternately and integrally layered, is put to practical use (see, for example, Patent Documents 1 and 2). In the multilayer wiring board, a conductor layer below a resin insulating layer and a conductor layer above the resin insulating layer are connected through a via conductor formed inside of the resin insulating layer.
Via conductors to connect each conductor layer include a filled via conductor and a conformal via conductor. The filled via conductor is a type of via without a depressed portion where a via hole formed in a resin insulating layer is completely filled by a plating layer. On the other hand, the conformal via conductor is a type of via where a plating layer is formed along a shape of a via hole and therefore the via hole is not completely filled up with the plating layer, thus the conformal via conductor has a depressed portion.
In a multilayer wiring board disclosed in Patent Document 1, the resin insulating layer has a hourglass-shaped via hole that penetrates the resin insulating layer to have a reduced diameter at the center in the thickness direction. A filled via conductor is formed to fill up this via hole. In a multilayer wiring board disclosed in Patent Document 2, the resin insulating layer has a via hole with an inverted truncated cone shape that penetrates to decrease in diameter from a top surface toward a bottom surface. A conformal via conductor is formed in the via hole. Then, the conformal via conductor has a depressed portion inside which a part of the resin insulating layer at an upper layer side is filled up.
The multilayer wiring board of Patent Document 2 includes a core substrate with a front face and a reverse face. On these surfaces of the core substrate, buildup layers, which are multilayered by alternately layering a plurality of resin insulating layers and a plurality of conductor layers, are formed. In this multilayer wiring board, the core substrate is formed to be thick, compared with the resin insulating layer constituting the buildup layer. In view of this, the core substrate has a straight through hole formed by drilling, and a through-hole conductor is formed inside of this through hole. Then, the through-hole conductor electrically connects the conductor layers formed on the front face and the reverse face of the core substrate. The through-hole conductor is internally filled with a filler such as epoxy resin.